# Step-by-Step:
01 - In a large bowl, combine bread flour, granulated sugar, and fine sea salt. Add cold milk, sourdough starter, and softened butter. Mix until just combined, then knead for approximately 8 minutes until a smooth, elastic dough forms.
02 - Shape dough into a rectangle, wrap tightly in plastic wrap, and refrigerate for at least 2 hours.
03 - Place 1 cup plus 2 tablespoons cold butter between two sheets of parchment paper. Pound and roll into an 8 by 6 inch rectangle. Chill until firm but still pliable.
04 - On a lightly floured surface, roll dough to a 16 by 8 inch rectangle. Position butter block on one half of the dough, fold the other half over it, and seal all edges firmly.
05 - Rotate dough 90 degrees and roll into a 24 by 8 inch rectangle. Fold into thirds like a business letter, wrap in plastic, and chill for 1 hour.
06 - Repeat the rolling and letter-fold process two additional times, chilling for 1 hour between each turn.
07 - After the final fold, wrap the dough and refrigerate overnight for 8 to 12 hours to develop flavor and allow proper lamination.
08 - The next morning, roll dough to a 24 by 12 inch rectangle, approximately 3/16 inch thick. Cut into 12 long triangles using a sharp knife.
09 - Starting from the wide end of each triangle, roll tightly into a crescent shape, tucking the tip underneath. Arrange on parchment-lined baking sheets.
10 - Cover loosely and allow to proof at room temperature until doubled in size, approximately 2 to 3 hours.
11 - Heat oven to 400°F (200°C).
12 - Whisk together the large egg and milk. Lightly brush the egg wash mixture over each croissant.
13 - Bake for 18 to 22 minutes until golden brown and crisp. Cool slightly before serving.